X-Ray Studies of Gallium Arsenide-Silicon Heterostructures.

Gallium arsenide-silicon heterostructures combine complementary electronic and structural properties to greater technological potential. The large 4% mismatch in lattice parameter between the two semiconductors strongly affects disorder within the film and at the interface. We employ x-ray scattering to explore the structural properties of these layered materials grown by molecular beam epitaxy. The thermal expansion of thick GaAs films on nominal Si(001) is measured with x-ray diffraction. The gallium arsenide film exhibits an anisotropy between the in-plane and out-of-plane lattice parameters due to the large thermal mismatch and the interfacial constraint. Thick films are found to be strain-free at the growth temperature and expanded in-plane at room temperature. This contrasts with the in-plane room-temperature contraction shown in thin films due to the residual strain present during growth. Interface roughness in thin gallium arsenide films (a few hundred angstroms thick) on silicon substrates is measured with x-ray reflectivity. The roughness at the GaAs/Si interface is found to be approximately 9A for a (001) orientation miscut by 4^circ . Ex situ in vacuo annealing just below the growth temperature, while reducing both film disorder and the roughness of the GaAs surface, does not substantially reduce interface roughness. Slow heating just above the growth temperature degrades the overall film quality. We also study interface roughness in GaAs films grown on very thin films of Si on a GaAs substrate. The silicon thickness ranges from pseudomorphic (that is, in -plane lattice parameter matching the underlying GaAs) to non-pseudomorphic. The GaAs/Si interface roughness increases from approximately 5A for the thinnest Si interlayer. The GaAs surface roughness increases dramatically at about the critical film thickness. But the onset of increased surface roughness occurs well before the Si thickness approaches critical and before the onset of increased interface roughness.